H3C Technologies H3C S5560 Series Switches - Manual (Page 47)

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a. DHCP server compares the client against DHCP user classes in the order they are configured.
b. If the client matches a user class, the DHCP server selects an IP address from the address range
of the user class.
c. If the matching user class has no assignable addresses, the DHCP server compares the client
against the next user class. If all the matching user classes have no assignable addresses, the
DHCP server selects an IP address from the common address range.
d. If the DHCP client does not match any DHCP user class, the DHCP server selects an address in
the IP address range specified by the address range command. If the address range has no
assignable IP addresses or it is not configured, the address allocation fails.
NOTE:
All address ranges must belong to the primary subnet. If an address range does not reside in the primary
subnet, DHCP cannot assign the addresses in the address range.
• Method 2—Specify a primary subnet and multiple secondary subnets in an address pool.
The DHCP server selects an IP address from the primary subnet first. If there is no assignable IP
address in the primary subnet, the DHCP server selects an IP address from secondary subnets in the
order they are configured.
Principles for selecting an address pool
The DHCP server observes the following principles to select an address pool for a client:
1. If there is an address pool where an IP address is statically bound to the MAC address or ID of the
client, the DHCP server selects this address pool and assigns the statically bound IP address and
other configuration parameters to the client.
2. If the receiving interface has an address pool applied, the DHCP server selects an IP address and
other configuration parameters from this address pool.
3. If no static address pool is configured and no address pool is applied to the receiving interface, the
DHCP server selects an address pool depending on the client location.
{
Client on the same subnet as the server—The DHCP server compares the IP address of the
receiving interface with the primary subnets of all address pools.
− If a match is found, the DHCP server selects the address pool with the longest-matching
primary subnet.
− If no match is found, the DHCP server compares the IP address with the secondary subnets
of all address pools. It selects the address pool with the longest-matching secondary subnet.
{
Client on a different subnet than the server—The DHCP server compares the IP address in the
giaddr field of the DHCP request with the primary subnets of all address pools.
− If a match is found, the DHCP server selects the address pool with the longest-matching
primary subnet.
− If no match is found, the DHCP server compares the IP address with the secondary subnets
of all address pools. It selects the address pool with the longest-matching secondary subnet.
For example, two address pools 1.1.1.0/24 and 1.1.1.0/25 are configured but not applied to any DHCP
server's interfaces.
• If the IP address of the receiving interface is 1.1.1.1/25, the DHCP server selects the address pool
1.1.1.0/25. If the address pool has no available IP addresses, the DHCP server will not select the
other pool and the address allocation will fail.